With high traffic sites there are many things that could cause your spikes and load issues such as email, log parsing, mysql or other things pop up at times. I doubt apache has a performance problem but there is no way to tell for sure without more information. I also have many high traffic servers pumping out 4-6 TB monthly as well.

Also try increasing your max clients to 500, 300 is a bit low for a high traffic box and check unique connections for any aggressive users.